1. Field of the Invention
The present invention relates generally to the navigation of remote controlled vehicles. More particularly, it concerns the navigation of remote controlled vehicles through the combined use of differential GPS and laser tracking along with compass navigation and/or wheel encoders. Even more particularly, it concerns navigation to maintain an echelon formation of robotic vehicles useful in landmine clearing applications.
2. Description of Related Art
Accurate navigation of remote controlled vehicles is important in many fields. For instance, in landmine clearing applications, one typically employs a formation of robotic, remotely controlled vehicles in echelon formation. Maintaining a proper, tight echelon formation requires accurate navigation of each vehicle relative to other vehicles. Likewise, the ability to accurately mark the location of mines requires accurate navigation. Sometimes accuracy to within 10 cm is required.
To achieve accurate navigation, many systems rely upon the Global Positioning System (GPS) and, particularly, differential GPS (DGPS). Although GPS provides an extremely useful navigation technique, it unfortunately can experience dropouts, effectively leaving no or insufficient navigation signal for a period of time. During a dropout period, remotely controlled vehicles may become misguided and/or fall out of formation. In applications such as landmine clearing, this is unacceptable.
To compensate for its limitations, GPS is sometimes replaced in favor of other navigation techniques: “dead reckoning,” which typically relies on inertial measurements, ground speeds, etc.; landmarks; celestial objects; the Omega Radio signal based worldwide navigation system; LORAN-C (LOng-RANge navigation), which is radio range finding along coastal areas; SatNav, which is a satellite-based radiodetermination system; or Sonar detectors.
Unfortunately, each of these methods suffers from drawbacks. Systems relying upon dead reckoning/inertial navigation systems sometimes suffer because navigation accuracy is usually time-dependent due to inherent continuous drift characteristics. The longer inertial navigation systems are used on their own, the greater the error associated with drift. Accordingly, the error may become so great as to overrun a maximum required navigational accuracy. Landmark navigation is highly dependent upon local area or horizon landmarks and/or reflectors. Celestial navigation typically determines position determination based on “shooting” known reference stars; this technique, however, is complicated and only works at night in good weather with limited precision. Omega is based on relatively few radio direction beacons, its accuracy is a disappointing 4 to 7 kilometers, and OMEGA station maintenance was discontinued in late 1997. LORAN-C suffers from limited coverage (mostly coastal) and is based upon radio signal timing, exhibiting an accuracy of only 20 to 100 meters. SatNav is based on low-frequency doppler measurements, so it is sensitive to small movements at a receiver. Its accuracy is only 10 to 50 meters, and although its coverage is worldwide, it is non-continuous. Finally, sonar detectors are useful for obstacle detection but not positioning.
Other potentially useful navigation techniques include laser tracking systems, compass-based navigation, and wheel encoders. Laser tracking systems are typically based on laser radar position measuring techniques and provide very accurate distance measurement, coupled with a gimbled axis for azimuth direction information. Compass-based and wheel encoder-based navigation systems are, in turn, well-suited for providing very accurate angular or directional information. Because these systems offer respective advantages, some remote controlled vehicles may utilize one of the systems to achieve navigation. Despite their advantages, existing navigation systems do not utilize these techniques in combination with GPS in such a way to eliminate problems associated with GPS dropout, as disclosed herein.
The referenced shortcomings of conventional methodologies mentioned above are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning the navigation of remote controlled vehicles. Other noteworthy problems may also exist; however, those mentioned here are sufficient to demonstrate that methodology appearing in the art have not been altogether satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.